Blood pressure is the force within the arterial system of an individual that ensures the flow of blood and delivery of oxygen and nutrients to the tissue. Prolonged reduction or loss of pressure severely limits the amount of tissue perfusion and could therefore result in damage to or even death of the tissue. Although some tissues can tolerate hypoperfusion for fairly long periods of time, the brain, heart and kidneys are very sensitive to a reduction in blood flow. Thus, during surgery, blood pressure is a frequently monitored vital sign. During and after surgery, blood pressure is affected by the type of surgery and physiological factors such as the body's response to the surgery. Moreover, during and after surgery, blood pressure is manipulated and controlled using various medications. Often, these physiological factors and the given medications result in a situation requiring immediate blood pressure measurement, and corrective action.
The most commonly used method of controlling an individual's blood pressure, particularly during surgery and in the critical period following surgery, is with vasoactive medications. These agents control the individual's blood pressure primarily by altering the resistance to blood flow in the peripheral arteries. For example, a vasodilating agent reduces peripheral resistance by increasing arterial compliance, and thereby reduces blood pressure. Conversely, a vasoconstrictor increases peripheral resistance by decreasing arterial compliance, and thereby increases blood pressure. Alternatively, inotropic agents can adjust the strength of the heart's contractions to modify blood pressure.
In some clinical situations, dramatic changes in blood pressure can occur instantaneously. For example, a sudden change in pressure may occur due to a reaction to drug therapy. Also, patient reactions to the surgery, sudden occlusion of an artery due to an embolism, or even sudden cardiac arrest are a few of the possibilities. It is very important to detect these sudden changes immediately, and to insure that the direction and amount of the changes be accurate within certain limits. Conversely, it is equally important that false indications of significant blood pressure changes do not occur.
Because of the patient's changes in blood pressure, it is important to constantly monitor blood pressure. The traditional method of measuring blood pressure is with the use of a stethoscope, occlusive cuff and pressure manometer. However, this technique is slow, subjective in nature, requires the intervention of a skilled clinician and does not provide the timely readings frequently required in critical situations.
For these reasons, two methods of measuring blood pressure have been developed: noninvasive, intermittent methods that use an automated occlusive cuff device; and invasive, continuous (beat-to-beat) measurements.
The noninvasive cuff method does not have the inherent disadvantages of the invasive technique including the risk of embolization, nerve damage, infection, bleeding and vessel wall damage. It also does not provide the continuous beat-to-beat pressure variations obtainable with the invasive method. Further, the noninvasive cuff method typically requires 15 to 45 seconds to obtain a measurement, and since it is an occlusive technique, the method should allow a minimum of 15 seconds to ensure sufficient venous recovery. Thus, at best there is typically 1/2 to 1 minute between updated pressure measurements. When fast acting medications are administered, this is an inordinately long amount of time to wait for an updated pressure reading. Also, frequent cuff inflations over extended periods of time may result in ecchymosis and/or nerve damage in the area underlying the cuff.
Several systems have been developed to address the need for continuous, noninvasive blood pressure measurement. These systems are described below.
The following patents show a technique known as photoplethysmography. Wesseling et al., European Patent Document 0048060 (1982), and U.S. Pat. Nos. 4,406,289 (1983), 4,510,940 and 4,539,997 (1985); Hyndman, U.S. Pat. No. 4,475,554 (1984); Kisioka, U.S. Pat. No. 4,524,777 (1985); Sun, U.S. Pat. No. 4,846,189 (1989); and Penaz, U.S. Pat. No. 4,869,261 (1989), all relate to this technique. They are commercially implemented in a device known as the Finapres.
The Finapres uses a small inflatable air cuff containing an infrared photoplethysmograph. The cuff is applied to one of the subject's fingers or thumb, and the plethysmograph measures the absorption at a wavelength specific for hemoglobin. The device first measures the individual's mean arterial pressure, and then varies the cuff pressure around the finger to maintain the transmural pressure at zero as determined by the plethysmograph. The device tracks the intra-arterial pressure wave by adjusting the cuff pressure to maintain the optical absorption constant at all times.
There are three major disadvantages to this technique. First, when there exists peripheral vasoconstriction, poor vascular circulation, or other factors, the blood pressure measured in a finger is not necessarily representative of central blood pressure. Second, the signal amplitude detected by the photoplethysmograph is a function of the changes in the diameter of the artery within the finger, and is determined by the compliance characteristics of the artery. The device maintains this amplitude at a constant value. This value, or set point, must correspond to the point of zero transmural stress in order to determine the correct pressure. During surgery for example, the device cannot differentiate between changes in photoplethysmograph amplitude due to intra-arterial pressure changes and those due to arterial wall compliance changes. Consequently, the Finapres cannot accurately respond to pressure changes caused by changes in vasomotor tone. Third, maintaining continuous cuff pressure causes restriction of the circulation in the finger being used, which is uncomfortable when maintained for extended periods of time such as during surgery or during a stay in an acute care unit.
Russel, U.S. Pat. Nos. 4,669,485 (1987), 4,718,426, 4,718,427 and 4,718,428 (1988), show a device using a conventional blood pressure cuff applied to a person's upper arm to sense an oscillometric signal. The subject's blood pressure is obtained initially by the oscillometric technique, and then changes in the oscillometric signal indicate changes from this initial reference pressure.
There are two major drawbacks to this technique. First, the use of a large air bag as the sensing device provides a means for detecting the fundamental and lower harmonics of the blood pressure signal (up to a few Hertz), but also acts to attenuate many higher order harmonics containing key information relating to blood pressure variations. Second, the use of a cuff to detect the oscillometric signal creates a signal that is very sensitive to patient movement. Since patient movement is often encountered during surgery or in critical care situations, the device requires frequent recalibration to be accurate.
Eckerle, U.S. Pat. Nos. 4,269,193 (1981), 4,799,491 and 4,802,488 (1989); Newgard, U.S. Pat. No. 4,423,738 (1984); Yokoe et al., U.S. Pat. No. 5,033,471 (1991); and Shinoda et al., U.S. Pat. No. 5,165,416 (1992), describe sensing means for detecting the pressure wave in the underlying artery of an individual using a technique known as the tonometric technique.
A commercial implementation of this technique is a device manufactured by Nippon Colin. This device uses a multi-element piezoresistive sensor to noninvasively detect the blood pressure wave at the radial artery. This signal is then processed and changes in its amplitude are used to interpret changes to the pressure values obtained using the conventional oscillometric technique.
The major drawback to this technique lies in the method of interpreting changes to the waveform signal. Reliance solely on amplitude changes is misleading since the signal amplitude may increase or decrease with an increase in blood pressure, etc. Secondly, it is dependent on the artery being exactly flat, and variations in artery flatness can introduce errors. It also assumes that the selected sensing element is small with respect to the artery, and that it does not move from its position centered over the artery. Thus, any movement such as that often encountered in surgery or critical care situations will reduce the accuracy of this device.
Smith, European Patent Document 0 443 267 A1 (1991), describes a technique using changes in pulse transit time to provide a continuous, noninvasive measure of blood pressure. This technique was developed by Sentinel Monitoring, Inc., of Indianapolis, Ind., and uses a duplicity of photometric sensors similar to those used with oximeters. Typically, one sensor is applied to the subject's ear lobe, and the other to a finger. The sensors are used for determining changes in the arrival time of the pulse at each of these sites, and to determine changes in local blood volume. Following an initial calibration pressure measurement obtained with a conventional blood pressure cuff, the Smith device adjusts these pressures by interpreting changes in the pulse transit time and in the optical density of the photoplethysmograph signal.
There are two disadvantages to the Smith technique. First, changes in pulse transit time are very small along major arteries. As a result, small errors caused by patient movement or noise render questionable data. Second, small variations in photoplethysmographic waveform morphology or sensor noise can generate measurement errors greater than the sensitivity of the technique to changes in blood pressure.
Gordon, et al., U.S. Pat. No. 4,960,128 basically shows a method of determining blood pressure by measuring a single harmonic of the frequencies and displacements of the patient's arterial wall. In Gordon, initial (absolute) blood pressure values are obtained with a cuff and stethoscope or via an intermittent automated cuff machine, and manually entered into the device as initial reference values. A continuous sensor signal is supplied by a noninvasive sensor attached to the patient's skin above an artery. The sensor signal is filtered, amplified and then sampled. This time sampled sensor data is then Fourier transformed into the frequency domain and normalized at 1024 point intervals.
As blood pressure changes, the reported frequencies and their relative amplitudes change. A comparison is made between the fundamental frequency of the present signal and the initial signal. For each shift in frequency (+or -) of 1 Hz, the offset is adjusted correspondingly to yield a change of 5 mm Hg. Thus, Gordon shows a device in which the patient's blood pressure is determined based on the difference in position of the fundamental frequency of the sensor signal and initial signal.
The technique described in the Gordon reference does not adequately account for the plurality of factors that can reflect a change in blood pressure. There is a multitude of waveshapes that can accompany a given set of blood pressure values, and the Gordon technique is limited by its function of comparing the frequency with the maximum amplitude of the current signal to that of the initial signal to determine blood pressure.